Toward epsilon levels of measurement precision on 234U/238U by using MC-ICPMS

Variations in U-234/U-238 have wide-ranging applications as tracers for ground- and river-water fluxes and is an essential component in U-series dating. Analytical developments for measuring U-234/U-238 have progressed from direct alpha-counting, with precisions at the percent level, to thermal ionization and multiple-collector inductively coupled plasma mass spectrometry (TIMS and MC-ICPMS, respectively) isotopic measurement techniques. However, U-234/U-238 is difficult to measure with better than permil precision because of the small atomic ratios for most geological samples (similar to10(-4) range). Using a Nu Instruments Nu Plasma MC-ICPMS, we have developed two analytical techniques for the precise measurement of U-234/U-238: (1) a conventional standard-bracketing protocol using multiple Faraday cups and electron multipliers with ion counting capabilities (FM) and (2) a standard-bracketing Faraday cup protocol (FF). Both are capable of measuring U-234/U-238 with precisions at the epsilon level (1 epsilon = 1 part in 10(4)): (1) The conventional standard-bracketing FM measurements are conducted as static measurements with the minor U-234 isotope measured in a conventional discrete dynode electron multiplier (SEM) equipped with ion counter and a retardation filter. The Faraday-multiplier gain is measured using bracketing measurements of the U metal standard CRM-145. The external reproducibility of U-234/U-238 (reformulated into delta-notation as delta(234)U), interspersed with frequent measurements of the gain, is at the +/-0.6% level (2sigma) for both uraninite and carbonate standards. takes similar to75 min and consumes similar to120 ng of U per measurement. (2) The static standard-bracketing FF protocol measures all three natural U isotopes in Faraday collectors. This is not usually possible using a standard multiple-Faraday array due to the large differences in the abundances of naturally occurring U isotopes. In our study, this is achieved by replacing the standard 10(11) Omega resistor for the U-238 Faraday cup with a 10(9) Omega resistor. The 10(9) Omega resistor enables the measurement of ion beams that are similar to100 times larger than can be accommodated by the normal 10(11) Omega resistor, so U-238 and U-234 are measured simultaneously in Faraday cups with intensities of similar to9 x 10(-9) and similar to5 x 10(-13) A, respectively. All measurements are normalized to bracketing CRM-145 standard measurements (measured with similar U-238 signal intensities) thereby correcting for significant tailing from the large U-238 ion beam below the smaller U-234 beam. Measurements are conducted over 2 min on-peak and 400-650 ng of U-238 is required per analysis. External reproducibility for samples with low matrix/U ratios (e.g., uraninites) is better than +/-0.3% (2sigma). Coral samples show a slightly poorer external reproducibility of +/-0.4% (2sigma) due to a higher matrix/U ratios of these samples. Repeat measurements of CRM-145 give respective delta(234)U values of -36.44 +/- 0.10% (2sigma(m), n = 9) and -36.50 +/- 0.14% (2sigma(m), n = 54) using the FF and FM analytical technique, assuming Harwell uraninite (HU-1) is in secular equilibrium with respect to U-234/U-238. The improved analytical precision achieved in this study for 234U/238U measurement is superior to any other reported measurements and is of great importance for U-series dating errors, particularly for samples older than 300,000 years before present. (C) 2004 Elsevier B.V. All rights reserved.